1. Field of the Invention
The present invention relates to an angle of rotation sensor such as a steering angle sensor. More particularly, the present invention relates to an optoelectronic angle of rotation sensor having an illuminated encoder disk that carries an analog coding and is coupled to the rotational movement of a rotor, and having a sensor array being arranged with respect to its longitudinal direction transversely to the movement direction of the encoder disk, wherein individual transducer elements of the sensor array sense the coding of the encoder disk.
2. Background Art
Angle of rotation sensors are frequently used for automatic positioning processes and measurements in machine tools and coordinate measuring devices. In the automotive industry, angle of rotation sensors are used to determine the absolute angle of rotation of a steering wheel. Sensors of this type are referred to as steering angle sensors. In motor vehicles, the steering angle value may be required for acting upon a dynamic control system. In addition to the steering angle value, a dynamic control system also receives other measurement data such as the rotational speed of the wheels or the rotation of the motor vehicle about its vertical axis. The dynamic control system evaluates the absolute steering angle deflection and the steering speed together with other acquired data for controlling actuators such as brakes and/or for the engine management system.
DE 40 22 837 discloses an optoelectronic steering angle sensor having a light source and a line sensor arranged in parallel at a certain distance from one another. An encoder disk (i.e., code disk) arranged between the light source and the line sensor is connected to the steering spindle to rotate with the steering spindle. In this case, the line sensor is a charge coupled device (CCD) line sensor having transducer elements. An Archimedean spiral extending over 360xc2x0 realized in the form of a light slit is used as the coding on the code disk. Information on the actual steering angle position is obtained based on the illumination of the corresponding transducer elements of the line sensor at a certain steering deflection.
The Archimedean spiral used as the coding is realized continuously such that the coding represents an analog coding. In this angle of rotation sensor, only a fraction of the transducer elements in the line sensor participate in determining the angular position of the steering wheel. Namely, only the transducer elements that are illuminated through the light slit. The remaining transducer elements of the line sensor participate in evaluation of the angle information only indirectly because the transducer elements that remain dark make it possible to determine only that the angular position of the steering wheel is not located in those angular regions represented by the non-illuminated transducer elements. One problem of such a coding can be seen in that the coding which is realized in the form of a light slit may become contaminated, for example, by debris such as hair, such that detection of the steering angle is no longer possible in this coding region. The contamination sectionally shades the coding and the transducer elements to be illuminated in order to detect this angular position remain dark. Consequently, this angular position is not defined.
In view of the prior art, it is an object of the present invention to provide an optoelectronic angle of rotation sensor such that a reliable angle determination is possible with partially contaminated coding.
The present invention attains this object as the angle coding reproduced on a sensor array essentially acts upon all sensor array transducer elements such that the angle information sensed by the sensor array is proportionally contained in every transducer element. The angle coding represents the reproduction of an optical oscillation structure on the sensor array extending in the longitudinal direction of the sensor array and changes with respect to at least one parameter in the movement direction of the code disk. A frequency-related analysis of the intensity signal detected by the transducer elements is carried out to decode the optical information detected by the sensor array with respect to the angle information contained therein.
In the angle of rotation sensor according to the present invention, which is suitable as a steering angle sensor for determining the absolute angle of rotation of a steering wheel of a motor vehicle, essentially the entire sensor array formed by the transducer elements is used for determining the respective angle information. The angle information is proportionally contained in every transducer element which participate in the evaluation of the angle information. For example, if the sensor array such as a line sensor has 128 transducer elements the angle information to be sensed is proportionally contained in each of the 128 transducer elements. If a reference track is provided on the encoder disk in addition to an angle coding, one region of the sensor array is reserved for reproducing the reference track. The angle information is then reproduced using the remaining transducer elements of the sensor array.
Determination of the angle coding is possible in instances where individual transducer elements are not illuminated, for example, due to contamination of the encoder disk, and consequently cannot contribute proportional angle information. Determination of the angle coding is possible because it can be assumed that the number of transducer elements remaining dark due to contamination is much smaller than the number of illuminated transducer elements. The number of illuminated transducer elements is, however, sufficiently large for reconstructing the optical oscillation structure to obtain the angle information contained therein.
In order to realize this measurement principle, the coding of the angle of rotation sensor according to the present invention is realized in the form of a reproduction of an oscillation structure in the sense of an optical transmission modulation extending in the longitudinal direction of the sensor array. This optical oscillation structure continuously changes with respect to one of its parameters (such as frequency or phase) in the movement direction of the encoder disk such that an angle detection is possible as a function of change of this parameter. Decoding of the angle information detected by the sensor array and present in the form of an optical oscillation structure preferably takes place in the form of frequency analysis of the amplitude signal progression over all transducer elements.
A frequency analyzer or a phase-lock loop (PLL) may be used to carry out the frequency-related analysis. The frequency analysis itself may be realized in the form of a Fourier transformation such that a spatial frequency spectrum can be assigned to a certain rotor position.
In a preferred embodiment of the present invention, the optical oscillation structure is mono-frequent in any angular position. In order to realize such encoding on an encoder disk, the frequency of the structure may change in the movement direction of the encoder disk. In order to reproduce the oscillation structure on the sensor array, one embodiment of such an angle of rotation sensor contains an encoder disk with a transmission that is sinusoidally modulated in the radial direction. The modulation frequency of the transmission continuously changes in the movement direction and/or the phase position of the transmission changes over the measurement range. This coding, which is reproduced on the sensor array, exhibits a continuous change from bright regions to dark regions that corresponds to the frequency, wherein a continuous transition is also provided with respect to the intensity.
According to another embodiment of the present invention, such an optical oscillation structure may be realized by designing the coding of the encoder disk in the form of a mask having bright and dark strips, wherein this coding is diffusely illuminated to such a degree that the bright/dark transitions of the mask extend over numerous transducer elements. In this case, the edges of the optical oscillation are realized by these transition regions.